Liquid crystal display devices have been used in various information display terminals and video devices. The major operating system for the liquid crystal display device is the twisted nematic ("TN") mode and the super twisted nematic ("STN") mode. Though they are commercially used in the market at present, the problems of narrow viewing angle are still remained unsolved.
An In-Plane Switching ("IPS") mode liquid crystal display has been suggested to solve foregoing problems.
As described in FIG. 1, a plurality of gate bus lines 11 are formed on a lower insulating substrate 10 along an x direction shown in the drawings and the gate bus lines 11 are parallel to each other. A plurality of data bus lines 15 are formed along an y direction which is substantially perpendicular to the x direction. Therefore a pixel region is defined. At this time, a pair of gate bus lines 11 and a pair of data bus lines 15 are shown in the drawing so as to define the pixel region. The gate bus line 11 and the data bus line 15 are insulated by a gate insulating layer (not shown).
A counter electrode 12, for example in the form of a rectangular frame, is formed within the pixel region and it is disposed at the same plane with the gate bus line 11.
A pixel electrode 14 is formed at each pixel region where the counter electrode 12 is formed. The pixel electrode 14 consists of a web region 14a which divides the region surrounded by the rectangular frame shaped counter electrode 12 in the y direction, a first flange region 14b connected to one end of the web region 14a and simultaneously overlapped with the counter electrode 12 of the x direction, and a second flange region 14c which is parallel to the first flange region 14 and is connected to the other end of the web region 14a. That is to say, the pixel electrode 14 seems the letter "I". Herein, the counter electrode 12 and the pixel electrode 14 are made of opaque metal layers. To ensure an appropriate intensity of electric field, the width of both counter and pixel electrodes is preferably in the range of 10.about.20 .mu.m.
The pixel electrode 14 and the counter electrode 12 are insulated from each other by a gate insulating layer (not shown).
A thin film transistor 16 is disposed at the intersection of the gate bus line 11 and the data bus line 12. This thin film transistor 16 includes a gate electrode being extended from the gate bus line 11, a drain electrode being extended from the data bus line 15, a source electrode being extended from the pixel electrode 14 and a channel layer 17 formed on the upper portions of the gate electrode.
A storage capacitor (Cst) is disposed at the region where the counter electrode 12 and the pixel electrode 14 are overlapped.
Although not shown in FIG. 1, an upper substrate (not shown) equipped with a color filter (not shown) are disposed on the first substrate 10 opposite to each other with a selected distance. Herein, the distance between the upper substrate and lower substrate 10 is smaller than the distance between the counter electrode region in the y direction and the web region of the pixel electrode thereby forming an electric field which is parallel to the substrate surface. Further, a liquid crystal layer (not shown) having a plurality of liquid crystal molecules is interposed between the upper substrate (not shown) and the lower substrate 10.
Also, onto the resultant structure of the lower substrate and onto an inner surface of the upper substrate are formed homogeneous alignment layers respectively. By the homogeneous alignment layer, in the absence of electric field between the counter electrode 12 and the pixel electrode 14, long axes of liquid crystal molecules 19 are arranged parallel to the substrate surface. Also, by the rubbing axis of the homogeneous alignment layer, the orientation direction of the molecules 19 is decided. The reference R in the drawings means the direction of rubbing axis for the homogeneous alignment layer formed on the lower substrate 10.
A first polarizing plate (not shown) is formed on the outer surface of the lower substrate 10 and a second polarizing plate (not shown) is formed on the outer surface of the upper substrate (not shown). Herein, the first polarizing plate is disposed to make its polarizing axis to be parallel to the P direction of the FIG. 1. That means, the directions of rubbing axis R and polarizing axis P are parallel each other. On the other hand, the polarizing axis of the second polarizing plate is substantially perpendicular to that of the first polarizing plate.
When a scanning signal is applied to the selected gate bus line 11 and a display signal is applied to the data bus line 15, the thin film transistor 16 disposed adjacent to the intersection of the gate bus line 11 and the data bus line 15 is turned on. Then the display signal of the data bus line 15 is transmitted to the pixel electrode 14 through the thin film transistor 16. Consequently, an electric field E is generated between the counter electrode 12 where a common signal is inputted and the pixel electrode 14. At this time, as the direction of electric field E is referenced as x direction as described in the FIG. 1, it has a predetermined degree of angle with the rubbing axis.
Afterward, when no electric field is generated, the long axes of the liquid crystal molecules are arranged parallel to the substrate surface and parallel to the rubbing direction R. Therefore the light passed through the first polarizing plate and the liquid crystal layer, is unable to pass the second polarizing plate, the screen shows dark state.
On the other hand, when the electric field is generated, the long axes (or short axes) are rearranged parallel to the electric field, therefore the incident light passed through the first polarizing plate and the liquid crystal layer, passes the second polarizing plate, the screen shows white state.
At this time, the direction of the long axes of the liquid crystal molecules changes according to the electric field, and the liquid crystal molecules themselves are arranged parallel to the substrate surface. Accordingly, the viewer can see the long axes of liquid crystal molecules at all directions, and the viewing angle characteristic is improved.
However, the IPS mode liquid crystal display as described above also includes following problems.
It is well known that the refractive anisotropy (or birefringence, .DELTA.n) is occurred due to the difference in lengths of the long and the short axes. The refractive anisotropy .DELTA.n also varies according to the viewer's viewing directions. Therefore a selected color can be shown in the region where the polar angle is of 0 degree and the azimuth angle is in the range of degrees 0, 90, 180 and 270, even in the white state screen. This regards as color shift and more detailed description thereof is attached with reference to the equation 1. EQU T.apprxeq.T.sub.0 sin.sup.2 (2.chi.).multidot.sin.sup.2 (.pi..multidot..DELTA.nd/.lambda.) equation 1
wherein,
T: transmittance; PA1 T.sub.0 : transmittance to the reference light; PA1 .chi.: angle between an optical axis of liquid crystal molecule and a polarizing axis of the polarizing plate; PA1 .DELTA.n: birefringence; PA1 d: distance or gap between the upper and lower substrates (thickness of the liquid crystal layer); and PA1 .lambda.: wavelength of the incident light. PA1 an upper substrate and a lower substrate opposed to be spaced apart; PA1 a liquid crystal layer interposed between the upper and lower substrates, the liquid crystal layer including a plurality of liquid crystal molecules; PA1 a gate bus line and a data bus line formed on the lower substrate in a matrix configuration and defining unit pixel regions; PA1 a counter electrode disposed at a unit pixel region in an inner surface of the lower substrate; PA1 a pixel electrode overlapped with the counter electrode; PA1 a thin film transistor disposed at an intersection of the gate bus line and the data bus line; and PA1 homogeneous alignment layers formed on the inner surfaces of the upper and lower substrates respectively, the homogeneous alignment layers including rubbing axes respectively, PA1 wherein, an electric field disposed parallel to the gate bus line and another electric field disposed parallel to the data bus line are simultaneously formed in the unit pixel region when a voltage is applied to the pixel electrode, PA1 wherein, the counter and pixel electrodes are made of a transparent conductive material, a distance between the counter and pixel electrodes is smaller than a distance between the upper and lower substrates, widths of the counter electrode and the pixel electrode are set such that the liquid crystal molecules overlying the counter and pixel electrodes are sufficiently aligned by the electric field being generated between the counter and pixel electrodes. PA1 an upper substrate and a lower substrate opposed to be spaced apart; PA1 a liquid crystal layer interposed between the upper and lower substrates, the liquid crystal layer including a plurality of liquid crystal molecules; PA1 a gate bus line and a data bus line formed in the lower substrate in a matrix configuration and defining unit pixel regions; PA1 a counter electrode disposed at each unit pixel region, the counter electrode including: a body of a rectangular frame shape; a center bar disposed across the center of a body thereby dividing the body region into a first space and a second space; at least a first branch being disposed parallel to the gate bus line and dividing the first space; and at least a second branch being disposed parallel to the data bus line and dividing the second space; PA1 a pixel electrode overlapped with the counter electrode, the pixel electrode including: a first dividing electrode disposed between the body and the first branch and between the first branches and between the first branch and the center bar, and being parallel to the first branch; a second dividing electrode connecting one end of the first dividing electrode and being overlapped with the body; a third dividing electrode being disposed between the body and the second branch and between the second branches and being disposed parallel to the second branch; a fourth dividing electrode connecting one end of the third dividing electrode and being connected to the second dividing electrode and overlapped with the center bar; PA1 a thin film transistor disposed at an intersection of the gate bus line and the data bus line; and PA1 homogeneous alignment layers having rubbing axes respectively, disposed at inner surface of the upper and lower substrates, PA1 wherein the counter electrode and the pixel electrode are made of a transparent conductive material and a distance between the counter and pixel electrodes is smaller than a distance between the upper and lower substrates, widths of the counter electrode and the pixel electrode are set such that the liquid crystal molecules overlying the counter and pixel electrodes are sufficiently aligned by the electric field being generated between the counter and pixel electrodes. PA1 an upper substrate and a lower substrate opposed to be spaced apart; PA1 a liquid crystal layer interposed between the upper and lower substrates, the liquid crystal layer including a plurality of liquid crystal molecules; PA1 a gate bus line and a data bus line formed in the lower substrate in a matrix configuration and defining unit pixel regions; PA1 a counter electrode shaped of a rectangular plate and disposed at each unit pixel region; PA1 a pixel electrode overlapped with the counter electrode, the pixel electrode including: at least one first dividing electrode being extended to a selected portion of the counter electrode in a direction parallel to the gate bus line; a second dividing electrode connecting one end of the first dividing electrode; a plurality of third dividing electrode being extended to a selected portion of the counter electrode in a direction parallel to the data bus line; and a fourth dividing electrode connecting one end of the third dividing electrode and simultaneously being connected to the second dividing electrode; PA1 a thin film transistor disposed at an intersection of the gate bus line and the data bus line; and PA1 a homogeneous alignment layer having a rubbing axis disposed at each inner surface of the upper and lower substrates, PA1 wherein the counter electrode and the pixel electrode are made of a transparent material and widths of the pixel electrode and the counter electrode exposed by the pixel electrode are set such that the liquid crystal molecules overlying the counter and pixel electrodes are sufficiently aligned by the electric field being generated between the counter and pixel electrodes.
So as to obtain the maximum transmittance T, the .chi. should be .pi./4 or the .DELTA.nd/.lambda. should be .pi./2 according to the equation 1. As the .DELTA.nd varies with the birefringence difference of the liquid crystal molecules depending on the viewing directions, the value of .lambda. varies so as to make .DELTA.nd/.lambda. to be .pi./2. According to this condition, the color corresponding to the varied wavelength .lambda. appears in the screen.
Accordingly, as the value of .DELTA.n relatively decreases at the viewing directions "a" and "c" toward the short axes of the liquid crystal molecules, the wavelength of the incident light for obtaining the maximum transmittance relatively decreases. Consequently, a blue color having shorter wavelength than a white color can be looked in the screen.
On the other hand, as the value of .DELTA.n relatively increases at the viewing directions "b" and "d" toward the short axes of the liquid crystal molecules, the wavelength of the incident light relatively increases. Consequently, a yellow color having a longer wavelength than the white color can be looked in the screen.
Deterioration is caused in the resolution of IPS mode liquid crystal display.
Since the counter electrode 12 and the pixel electrode 14 of the IPS mode liquid crystal display are made of opaque metal layers, an aperture area of the liquid crystal display decreases, and the transmittance thereof also decreases. In addition, so as to obtain an appropriate brightness, a backlight with high intensity must be used and thus an electrical consumption increases, which is often undesirable.
To solve these limitations, a counter electrode 12 and a pixel electrode 14 made of a transparent material have been proposed. In such a liquid crystal liquid display the aperture ratio is often increased, but the transmittance is often not improved. To produce an in-plane electric field, the distance l between the electrodes 12 and 14 must often be set to be greater than the cell gap d. To obtain an appropriate intensity of the electric field, the electrodes 12 and 14 have relatively large dimension of width, for example, 10 to 20 .mu.m.
However, if the electrodes have such a large dimension of width, the liquid crystal molecules positioned right above the upper surfaces of the electrodes 12 and 14 do not move thereby forming equipotential lines. As the result, since the liquid crystal molecules positioned right above the upper surfaces of the electrodes continue to hold an initial configuration even in the presence of the electric field, the transmittance is little increased.